/Engine/Source/Runtime/Engine/Classes/Components/PoseableMeshComponent.h
Take the BoneSpaceTransforms array (translation vector, rotation quaternion and scale vector) and update the array of component-space bone transformation matrices (SpaceBases). It will work down hierarchy multiplying the component-space transform of the parent by the relative transform of the child. This code also applies any per-bone rotators etc. as part of the composition process
void FillComponentSpaceTransforms()